As a device for reducing frictional resistance, which reduces frictional resistance of a hull by blowing an air into the water of the outside of the hull, for example, there is a device described in Patent Document 1. This device for reducing the frictional resistance includes a chamber in which a plurality of blowing holes blowing an air into the water of the outside of the hull are formed. An air pressure inside the chamber is constant in every location, but a water pressure increases in the water outside the chamber along with an increasing of the water depth. Thus, when the diameter of the blowing holes is constant, the flow amount of the air blowing from the blowing hole placed in a deep position of the water depth is less than the flow amount of the air blowing from the blowing hole placed in a shallow position of the water depth. Accordingly, in the present invention, the opening area of the blowing hole placed in a deep position of the water depth is larger than the opening area of the blowing hole placed in a shallow position of the water depth, and thereby, the flow amount of the air blowing from each of the blowing holes is equalized.